Unable to add drive to raid

I have a 3 RAID arrays on my Linux server.

root@master:~# lsblk -b
NAME          MAJ:MIN RM           SIZE RO TYPE  MOUNTPOINTS
loop0           7:0    0    21474836480  0 loop  
loop1           7:1    0    10737418240  0 loop  
sda             8:0    0   512110190592  0 disk  
├─sda1          8:1    0        1048576  0 part  
└─sda2          8:2    0   512107741184  0 part  
  └─md0         9:0    0   511972474880  0 raid1 /
sdb             8:16   0   512110190592  0 disk  
├─sdb1          8:17   0        1048576  0 part  
└─sdb2          8:18   0   512107741184  0 part  
  └─md0         9:0    0   511972474880  0 raid1 /
sdc             8:32   0  3000592982016  0 disk  
└─sdc1          8:33   0  3000591450112  0 part  
  └─md127       9:127  0  9001368551424  0 raid5 
    └─vg0-lv0 252:0    0 11001479036928  0 lvm   /mnt/bigdrive
sdd             8:48   0  4000787029504  0 disk  
├─sdd1          8:49   0  3000591450112  0 part  
│ └─md127       9:127  0  9001368551424  0 raid5 
│   └─vg0-lv0 252:0    0 11001479036928  0 lvm   /mnt/bigdrive
└─sdd2          8:50   0  1000193654784  0 part  
  └─md126       9:126  0  2000116776960  0 raid5 
    └─vg0-lv0 252:0    0 11001479036928  0 lvm   /mnt/bigdrive
sde             8:64   0  4000787030016  0 disk  
├─sde1          8:65   0  3000591450112  0 part  
│ └─md127       9:127  0  9001368551424  0 raid5 
│   └─vg0-lv0 252:0    0 11001479036928  0 lvm   /mnt/bigdrive
└─sde2          8:66   0  1000193654784  0 part  
  └─md126       9:126  0  2000116776960  0 raid5 
    └─vg0-lv0 252:0    0 11001479036928  0 lvm   /mnt/bigdrive
sdf             8:80   0  4000787030016  0 disk  
├─sdf1          8:81   0  3000591450112  0 part  
└─sdf2          8:82   0  1000193654784  0 part  

md0 is a mirror raid and is irrelevant to my problem.

My other 2 raids are RAID 5 arrays.

root@master:~# cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4] [raid0] [raid10] 
md126 : active raid5 sdd2[0] sde2[1]
      1953239040 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/2] [UU_]
      bitmap: 0/8 pages [0KB], 65536KB chunk

md127 : active raid5 sdd1[1] sde1[2] sdc1[0]
      8790398976 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [UUU_]
      bitmap: 2/22 pages [8KB], 65536KB chunk

md0 : active raid1 sdb2[1] sda2[0]
      499973120 blocks super 1.2 [2/2] [UU]
      
unused devices: <none>

root@master:~# mdadm --detail /dev/md126
/dev/md126:
           Version : 1.2
     Creation Time : Tue May 21 19:23:02 2024
        Raid Level : raid5
        Array Size : 1953239040 (1862.75 GiB 2000.12 GB)
     Used Dev Size : 976619520 (931.38 GiB 1000.06 GB)
      Raid Devices : 3
     Total Devices : 2
       Persistence : Superblock is persistent

     Intent Bitmap : Internal

       Update Time : Tue Jun 11 19:07:38 2024
             State : clean, degraded 
    Active Devices : 2
   Working Devices : 2
    Failed Devices : 0
     Spare Devices : 0

            Layout : left-symmetric
        Chunk Size : 512K

Consistency Policy : bitmap

              Name : master:2  (local to host master)
              UUID : 266af7f7:8915136a:c8630864:2c514c63
            Events : 3201

    Number   Major   Minor   RaidDevice State
       0       8       50        0      active sync   /dev/sdd2
       1       8       66        1      active sync   /dev/sde2
       -       0        0        2      removed

root@master:~# mdadm --detail /dev/md127
/dev/md127:
           Version : 1.2
     Creation Time : Tue May 21 19:22:19 2024
        Raid Level : raid5
        Array Size : 8790398976 (8.19 TiB 9.00 TB)
     Used Dev Size : 2930132992 (2.73 TiB 3.00 TB)
      Raid Devices : 4
     Total Devices : 3
       Persistence : Superblock is persistent

     Intent Bitmap : Internal

       Update Time : Tue Jun 11 19:05:40 2024
             State : clean, degraded 
    Active Devices : 3
   Working Devices : 3
    Failed Devices : 0
     Spare Devices : 0

            Layout : left-symmetric
        Chunk Size : 512K

Consistency Policy : bitmap

              Name : master:1  (local to host master)
              UUID : cb895040:3fa006aa:88ea4323:15194452
            Events : 25871

    Number   Major   Minor   RaidDevice State
       0       8       33        0      active sync   /dev/sdc1
       1       8       49        1      active sync   /dev/sdd1
       2       8       65        2      active sync   /dev/sde1
       -       0        0        3      removed

As seen above, one array has 4 drives and one has 3. Both arrays are degraded with one drive missing each.

I have inserted a new drive "/dev/sdf" that has 2 partitions that exactly matches the sizes of the missing "drives".

The status of the 2 arrays looks identical to me.

Now, when I try to add the new/replaced drives to the 2 arrays, only one works (!!!).

root@master:~# mdadm --manage /dev/md127 --add /dev/sdf1
mdadm: add new device failed for /dev/sdf1 as 4: Invalid argument
root@master:~# mdadm --manage /dev/md126 --add /dev/sdf2
mdadm: added /dev/sdf2

I am even able to add the "/dev/sdf1" drive to the "/dev/md126" array (even though it's too big), so there is no problem with the drive itself.

I am really at a loss here. No idea why "/dev/127" refuses to accept the drive and throws "Invalid argument" without any further details to why...

UPDATE:

root@master:~# cat /sys/block/md126/md/dev-*/state | xargs
in_sync in_sync
root@master:~# cat /sys/block/md127/md/dev-*/state | xargs
in_sync in_sync in_sync

In dmesg, I see this error. No idea why it thinks it would have a superblock already as I am adding a new drive:

md: sdf1 does not have a valid v1.2 superblock, not importing!
md: md_import_device returned -22